In the continuous process of block or solution polymerization of styrene, the product polymer which is discharged from the polymerization reactor contains a high concentration of volatile matter, i.e., the monomer styrene or the solvent used for polymerization. A high-degree evacuation technique by the use of a vertical falling-film evaporator has been practiced to remove the volatile matter included in such a polymer composition.
In the conventional process employing this technique, the polymer composition is heated through a heater or heat exchanger in advance of the evaporation to offset the latent heat taken away by the evaporation. This method is, however, undesirable in point of polymer quality because the remaining monomer tends to form the polymer of low-molecular weight when exposed to high temperatures of the preheating. When the polymer composition of low temperature is fed to the evaporator, its viscosity increases with the vaporization of the solvent and/or monomer and it becomes liable to partly solidify on account of the temperature drop due to the vaporization, so that smooth revolution of the agitator becomes difficult.